In present day copying machines or electronic printer devices, the fixing or permanent adherence of a toner image on a substrate in the form of a sheet is often carried out with a so-called hot roller or nip-roller fixing system. This image-fixing device comprises a pair of rollers through the nip of which a sheet carrying a toner image is fed. The surface of the roller contacting the toner image is heated above the softening temperature of the toner resin, which becomes tacky or molten and forms a permanent bond with the surface of the sheet. The roller, contacting the toner image, is provided with a coating having abhesive (i.e. non-adhesive) properties versus the toner image. In order to increase the non-adhesive characteristics of the roller surface, use is often made of silicone oil. The rollers forming the image-fixing unit are pressed against each other. The roller contacting the backside of the sheet is generally covered with a silicone elastomer, capable of resisting the heat generated by the image-fixing roller.
Problems arise with hot roller image-fixing devices. In particular, in heavy duty printers where long periods between servicing are usual, it is difficult to maintain a constant image-fixing quality and a long roller lifetime.
A technique known as "flash-fixing" is also known in which a short intense burst of radiant energy is applied to the substrate carrying the toner image to be fixed. The wavelength of the radiant energy is chosen to be absorbed by the toner. Such a technique is unsuitable for multi-color images, where toners of different composition are carried on the substrate, said toners having different absorption characteristics in the spectrum.
A number of constructions of image fixing devices using infra-red radiant fixing have been proposed in the art. U.S. Pat. No. 3,449,546 (Dhoble/Xerox Corporation) describes a xerographic fusing apparatus, which is capable of heating toner powder to its glass transition point without damaging the paper support material, wherein the paper acts as a heat source to aid in the fusing process. U.S. Pat. No. 5,526,108 (Billet et al/Xeikon NV) describes a radiant fixing device comprising at least one radiant source, the peak energy output wavelength of which lies in the non-visible part of the spectrum.
Infra-red fixing devices however cause a loss of moisture from the substrate, as a result of the high temperature to which the substrate is heated, i.e. the substrate becomes too dry. This loss of moisture can result in deformation of the substrate and the low moisture level can result in the generation and retention of electrostatic charges on the substrate, both of which effects can produce problems in subsequent handling of the substrate.
A number of proposals have been made for fixing toner images by the use of a belt.
In U.S. Pat. No. 3,948,215 (Namiki/Ricoh Co., Ltd) a toner image on a support sheet is fused either by disposing the sheet in with a heating surface or by additionally pressing the sheet. After the toner particles are fused, the sheet is cooled while maintaining its image bearing side in contact with the surface previously used for heating, so that the particles solidify and the toner image is stiffened. Pressure may be applied to the support sheet as it contacts the heating surface.
U.S. Pat. No. 5,483,331 (Wayman et al./Xerox Corporation) describes a transfer and fusing belt arrangement in which three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which an electrically resistive substrate carrying toner images passes, with the toner images contacting the fusing belt. Electrical power is applied to the three fuser rollers so that only the portions between these rollers are heated.
U.S. Pat. No. 5,386,281 (Mitani et al./Hitachi Koki Co. Ltd) describes a thermal fixing device including a metal belt press contacted to a pressing roller. A heating device is associated with the belt. An image support carrying an unfixed toner image is passed through the nip between the roller and the belt so that the image is thermally fixed. The belt is also associated with a cooling device which cools the image support beyond the nip so that the image support can be stripped from the metal belt.
European patent application EP 0848304 (Xeikon NV), published Jun. 17, 1998, discloses a fixing device which comprises a belt in face-to-face contact with a reaction surface to form an extended contact zone therebetween. A substrate, carrying an unfixed toner particle image, is passed through the contact zone. The belt is heated adjacent the entrance to the contact zone to a temperature above the softening point of the toner. The belt is forcibly cooled intermediate the entrance and the exit to a temperature below the softening point of the toner. Pressure rollers apply pressure between the belt and the reaction surface intermediate the entrance and the exit. The belt is also heated adjacent the exit.
EP 0848304 also discloses an embodiment in which an unfixed toner particle image is transferred to a substrate in the contact zone. In the case of such transfer, it is recommended that the surface of the image should contact the substrate above the "fluid temperature", so as to ensure mixing of the toner particles of different colors. The "fluid temperature" is defined as the temperature at which the viscosity of the toner falls below 50 Pa s.
Due to the fact that dry toner images have a high thickness (sometimes more than 10 .mu.m), the appearance of such images is sometimes unnatural and non-uniform and these images usually have a non-uniform color saturation. While this appearance is acceptable for many applications, it is sometimes desired to provide an image having a different appearance or finish. By the term "finish" in the context of the present invention, we mean either a surface characteristic which is glossy, i.e. highly reflective, and/or which provides high saturation of colors, this usually being achieved by reducing the scattering of light from the surface of the printed article, or both such characteristics. A higher degree of color saturation can be very desirable in high quality print work.
It has been proposed to provide glossy images by the use of a toner which incorporates a glossing agent, or by the application of a transparent glossing layer over the toner image. However, these methods are costly in terms of consumables.
In U.S. Pat. No. 5,521,688 (Moser/Xerox Corporation) it has been proposed to provide glossy images by passing the substrate carrying the toner images through an oven heater to fix the images and then through a pair of glossing rollers operating at approximately the same temperature as the oven.
U.S. Pat. No. 5,319,429 (Fukuchi et al./Konica Corporation) describes a color printer having a fixer for fixing a toner image on a recording sheet, which includes an endless polyimide heat belt which is supported by a heat roller and a separation roller, and an endless conveyance belt which is supported by a pressure roller and another separation roller. The endless heat belt and the conveyance belt are pressed together over part of their length, so that a nip region is created between the first pair of rollers and the second pair. The belts have glossy surfaces. It is recommended in U.S. Pat. No. 5,319,429 that the most appropriate fixing condition can be obtained when the viscosity of the toner is about 5000 poise (=500 Pa s).
European patent application EP 0758766 (Xeikon NV) discloses an electrostatographic printer in which a web of receiving material, after passing through a fixing device, is contacted with a finishing element while at a temperature above the glass transition temperature of the toner to modify the finish of the toner image.
It would be desirable to use one and the same device to fix the toner images and to provide them with the desired gloss. However, contact-less fixing devices are unable to provide a uniform glossing effect, while we have found that the use of known heated rollers or heated belt fixing devices suffer from toner offset problems and do not provide sufficient control over the gloss and color saturation of the images. In particular such known devices exhibit limited process parameters, with a narrow window of optimum performance.
It is an object of the present invention to provide a device and method whereby un-fixed toner images can be fixed to a substrate and provided with a desirable level of gloss in one single device, while widening the range of operating conditions without risk of offset occurring.